As Pickles says. Just to be clear though, that is to the "old" Mandarin Oriental. Just in case you meant the Landmark Mandarin Oriental, that is even closer: just turn left out of Pacific Place onto Queensway at street level, and basically walk straight on. Cross over Queen's Road by the big HSBC building, and continue along Queen's Road on the other side. Across one more junction and the LMO is on your right. No more than 10 minutes walk.